Apparatus for erasing an optical image stored in a storage tube

ABSTRACT

An image storage tube is provided, which comprises a transparent back plate coated on one face with a storage layer made of a cathodochromic material and on the other with an electrically resistive coating; an electron gun including a cathode adapted to be maintained substantially at the earth potential and spaced from the back plate; and a apparatus for erasing an image stored in the storage layer which includes an image erasing power source for supplying a current to the resistive coating, a switch means for connecting the erasing power source with the back plate when energized, a high voltage generator for applying to the back plate a high voltage relative to the cathode, and control means for alternatively energizing the switch means and the high voltage generator.

United States Patent 1191 Uno 1 APPARATUS FOR ERASING AN OPTICAL IMAGE STORED IN A STORAGE TUBE [75] lnventor: Yoshlhiro Uno, Kadoma City, Japan [73] Assignee: Matsushita Electric Industrial Company, Limited, Kadoma City, Osaka, Japan 22 Filed: Oct. 6, 1971 21 Appl.No.: 187,084

Related US. Application Data [63] Continuation-impart of Ser. No. 830,438, June 4,

1969, abandoned.

[30] Foreign Application Priority Data June 7, 1968 Japan... 43-39789 July 8, 1968 Japan..'.-.,- 43-48615 [52] U.S. Cl. 315/10 [51] Int. Cl. H01] 31/26 [58] Field of Search 315/10, ll, 12

[56] I References Cited UNITED STATES PATENTS 3,594,607 7/1971 Frankland 315/12 3,683,358 8/1972 Eichelberger, 315/10 X 2,856,559 10/1958 Knoll 315/11 X 3,148,281 8/1964 Fyler 315/12 Mar. 12, 1974 3,239,766 3/1966 Manley 315/12 X 3,331,983 7/1967 3,500,116 3/1970 3,506,874 4/1970 3,564,321 2/1971 3,631,293 12/1971 3,641,555 2/1972 Griffin 315/12 X Primary Examiner-Carl D. Quarforth Assistant Examiner-P. A. Nelson Attorney, Agent, or Firm-John Lezdey [57] ABSTRACT An image storage tube is provided, which comprises a transparent back plate coated on one face with a storage layer made of a cathodochromic material and on the other with an electrically resistive coating; anelectron gun including a cathode adapted to be maintained substantially at the earth potential and spaced from the back plate; and a apparatus for erasing an' image stored in the storage layer which includes an image erasing power source for supplying a current to the resistive coating, a switch means for connecting the erasing power source with the back plate when en ergized, a high voltage generator for applying to the back plate a high voltage relative to the cathode, and

control means for alternatively energizing the switch means and the high voltage generator. 1

1 Claim, 2 Drawing Figures PAIENIEDHAR 12 1914 INVENTOR Y SH lHlRo vua BY M ATTOR v PAIENIEUIMR I 2 m4 3; 796; 907

IIIIIIIIIIIII I I I I I I I 5: ;Z ;E f* m3? C i I L I F HIGH FREQUENCY I J i 'LTER OSCILLATOR 5 55 i f b I I FLY BACK RECT'F'ER TRANSFOMER 5 G s4 INVENTOR ATTOR Y APPARATUS FOR ERASING AN OPTICAL IMAGE STORED IN A STORAGE TUBE This application is continuation-in-part of the copending application Ser. No. 830,438, filed on June 4, 1969, now abandoned.

This invention relates to an image storage tube, and more particularly to an image storage cathode-ray tube having an image storage layer of cathodochromic material and an apparatus for erasing an image stored in the storage layer.

An image storage tube having an image storage layer of cathodochromic material finds extensive applications for the purpose of storing optical signals for a prolonged period of time because of its inherent advantage of: great resolving power of the order of 1,000 lines per millimeters or more; compatibility with either the reflected or transmitted light to recognize the stored image; availability of projected images on an expanded scale; significantly reduced production cost; and capability of storing the information over the extended period of several weeks without critical deterioration in the quality of the image stored.

In contrast to these prominent features of the image storage tube of the type, problems are still experienced in erasing the optical information that has once been put in the storage plate. One of the practical methods proposed for erasing the stored information is the heating method in which the information is erased by heating the storage plate directly or indirectly, say, through application thereto of infrared radiation or by heating the foundation on which the storage plate is placed.

Cathodochromic material is a substance which will become opaque when bombarded with electrons. The application of heat to cathodochromic material restores the material to its normal transparent condition. Such materials practically used in the art include alkali halide, calcium fluoride, sodalite, calcium titanate and strontium titanate.

Such characteristics of cathodochromic material will be discussed in detail hereinbelow as to alkali halide crystal.

Pure alkali halide crystals are ionic crystals with facecentered cubic lattice which are transparent throughout the spectral range between the ultrared region of 4,000m,um and the ultraviolet region of 200m,um. If a lattice defect occurs in such crystals and an electron trapped at the lattice position corresponding to a halogen ion vacancy, then absorption of light of certain wavelength will result. The optical absorption of light of certain wavelength will result. The optical absorption of this type is ascribed to the creation of an F- center. The F-center can be induced, for instance, by irradiation of the named crystals with electron beam and are observed in a surface formed by vacuum evaporation of a fine polycrystalline alkali halide surface formed by evaporation in an atmosphere under reduced pressure. The maximum of the F-center optical absorption band (F-band) varies according to the type of the alkali halide used and ranges substantially throughout the visible spectrum; the F-band maximum of sodium chloride, potassium chloride and potassium bromide, for example, are known to occur at the wavelengths of 458mp.m, 556mum and 625mum, respectively, with the half-value width of about IOOmum.

The F-centers induced by irradiation of the evaporated fine crystalline surface with electron beams can be erased by heating the particular surface. Thus, the alkali halide crystalline surface is used as a storage surface utilizing the electron beams as a pen for writing in the given signals.

The tubes utilizing such phenomena are known as dark trace cathode-ray tubes or under the trade name of Skiatron.

Experiments conducted to evaluate the sensitivity and retention characteristics of the storage plate formed by vacuum evaporation with potassium chloride reveal that the optical density CD. of the storage plate is, insofar as the density is not very high, expressed as:

CD. log intensity of incident light/intensity of transmitted light B G O/0.001) U) where A: a constant dependent upon the type of the material to be evaporated and upon the condition of evaporation (if potassium chloride is evaporated at the angle of 45, then A 0.79);

B: a qonstant dependent upon the thickness of the evaporated film and the electron beam accelerating voltage;

Q: amount of charges;

t: time;

E and E constants (E 10.5 and E 0.36 if potassium chloride is used);

K: Boltzmann factor;

T: temperature, K.

lt is known from this equation and from experiments that a considerable amount of charge is required to attain a sufficient optical density of the image. If, for instance, the thickness of the image. If, for instance, the thickness of the potassium chloride film is 12.5 mum is 12.5m,u.m (i.c., 2.37mg/cm the angle of evaporation 45, and the electron beam accelerating voltage 27kV, then the electron beams must necessarily provide up to ZOnQ/crn to obtain a contrast ratio of 10 and up to IOOuQ/cm to obtain a contrast ratio of 100. Such requirements are easily met with present-day techniques of electron beam treatments.

Assuming, now, that an image stored in the storage plate is regarded as completely erased with the absorption density of the plate lowered to one hundredth of the initial value, then it will take about 20 seconds to have the image erased perfectly, provided the temperature of the storage plate is 300C, as will be ascertained from Equation 1.

A source of power required to energize the resistive coating of the back plate will be readily available so long as the potential at the storage plate is in the neighbourhood of the earth potential. In this case, however, the cathode of the electron gun is held at an extremely high negative voltage, say, in the range of 20kV to 30kV, with the result that the fabrication of the power source of the electron gun and the delivery of signals to the grid under a high negative voltage is difficult. It is advantageous for the potential at the cathode of the electron gun to be as near the earth potential as possible. This will result in a remarkable increase in the potential at the back plate, to a positive voltage of 20kV to 30kV. In order to heat the back plate when subjected to such an increased voltage, it is necessary either to temporarily cut off the high voltage and thereafter connect to the back plate the power source for erasing the image, or to protect the erasing power source from the high voltage in some other way.

The specific purpose of this invention is to provide an image storage tube with an improved image erasing apparatus which applies an image erasing power to the image storage tube with satisfied protection from the high voltage at the back plate of the tube.

In the drawings:

FIGS. 1 and 2 are diagrammatic views showing an image storage tube with an image erasing apparatus according to this invention.

First referring to FIG, 1, the image storage tube has an evacuated glass envelope enclosing therein a back plate 11 of glass or mica which is coated on one face with a transparent resistive material of stannic oxide or other suitable material, a storage layer 12 of potassium chloride carried by the back plate 11, refleeting mirrors 13 facing the storage layer 12 at a predetermined angle and an electron gun 14 for writing in the information supplied thereto. There are provided in the wall of the envelope 10 close to the storage layer 12 a pair of electrodes 15 and 15 which are connected electrically with the resistive coating of the back plate 11. As the coating of back plate becomes energized through the electrodes 15 and 15' heat is evolved therein and transferred to the storage layer 12, thereby erasing the F-centers that have been induced in the storage layer 12. The storage layer 12 being transluscent, it may lend itself to a storage tube of either lightreflecting or light transmitting type; the storage tube under discussion is illustrated to be of the latter type as an example. The mirrors 13 transfer the light from the light source 16 to the storage layer 12. The bandpass filter 17 may be so arranged as to have its band center coincident with the center of the F-center absorption band. The bandpass filter 17 may be dispensed with if desired.

The storage device further comprises, as customary, a focus coil 18 for focussing electron beam to a fine spot, deflection coil 19 for deflecting the electron beams focussed by the coil 18, and a projection lens 20 for projecting images from the storage layer 12 to the screen (not shown). A D.C. power source 22 holds the back plate at a high positive potential. Here, the storage layer can be regarded as playing the part ofa slide in the slide projector.

The storage layer 12 being formed of fine potassium chloride crystals of the order of lm .I.m, a great number of fine pieces of information can be written in the plate.

As shown, the image erasing apparatus comprises a switch means 25 inserted between the electrodes 15 and 15, an image erasing power source 26 connected with the contacts of the switch means, and a high voltage power source 22 for applying high voltage to the back plate of the tube. The switch means 25 has three pairs of contacts a and a, b and b, and c and c. The contacts a and a are for connecting the erasing power source 26 to the electrodes 15 and 15' to erase the image stored in the storage layer, while the contacts c and c are for energizing the back plate with high voltage supplied from the power source 22 to establish a high potential in the back plate relative to the electron gun. as will readily be understood from the illustration. The contacts b and b, on the other hand, are intended to keep the back plate disconnected from both of the power sources 22 and 26.

The power to be supplied from the power source may be either AC. or D.C. power or in the form of pulses. Where an Sn0 film is to be used as the resistive coating on the back plate, the use of an A.C. power will be preferred, partly because the SnO film would be free from deterioration resulting from the movement of ions therein, as is usually encountered where a D.C. power is employed, and partly because the voltage and current can be regulated easily with use of a transformer.

Although, furthermore, the power source 22 is herein illustrated to be constituted by a series of batteries, the power may more advantageously be supplied from a high frequency transformer such as of flyback type whereby the output of an oscillator set near the earth potential is Q stepped up in voltage and rectified into D.C. high voltage. In this instance, when the contact 0' is in its closed position the high voltage may be controlled by regulating the output of the oscillator. A detailed construction of such arrangement of the high voltage source is shown in FIG. 2.

FIG. 2 illustrates in more detail the image erasing apparatus which comprises two switches S and S simultaneously actuated by a solenoid 30 and each having stationary contacts connected to the electrodes 15 and 15'. A movable contact of the switch S, is connected to one terminal of an AC. erasing power source 26, the other terminal of which is connected to a movable contact of the switch S One terminal of the solenoid 30 is grounded and the other terminal of the solenoid 30 is connected to a stationary contact a of a switch S The switch 8:, further has stationary contacts b and c. The stationary contact c of the switch S is connected through a movable contact thereof to a positive terminal ofa D.C. power source 31, a negative terminal of which is grounded. The positive terminal of the D.C. power source 31 is further connected to a movable contact of a switch S which has three stationary contacts a, b and c. The stationary contact 0 of the switch S is connected to an input terminal 32 ofa high voltage generator 22'. An output terminal 33 of the high voltage generator 22' is connected to the electrode 15'. The switches S and S, are simultaneously actuated by suitable means.

The high voltage generator 22' comprises a high frequency oscillator 34 an output terminal of which is connected to an input terminal ofa flyback transformer 35. An output terminal of the flyback transformer 35 is connected to an input terminal of a rectifier 36. An output terminal of the rectifier 36 is connected to an input terminal of a filter 37, an output terminal of which is the output terminal 33 of the high voltage generator 22.

When, in operation, each movable contacts of the switches S and S is connected to each stationary contact c, the high frequency oscillator 34 is then energized by the D.C. power being supplied through the switch 8,, from the D.C. power source 31 thereby to produce a high frequency energy. The high frequency energy from the high frequency oscillator 34 is applied to the flyback transformer 35 which then steps up the high frequency energy into a high voltage energy. The high voltage energy is rectified by the rectifier 36 and thereafter applied to the filter 37. The filter 37 smoothes the rectified energy to produce a constant high voltage D.C. power on the output terminal 33, which high voltage power is applied to the electrode The high voltage DC. power establishes an electric field which is suitable for the image storing operation of the tube.

When the erasing operation is intended, the switches S and 5 are simultaneously changed over to make each movable contact connect to the stationary contact a. The high voltage generator 22 is then deenergized due to the disconnection of the switch S whereby the high voltage power disappears from the output terminal 33. The switch S completes a circuit from the positive terminal of the DC. power source 31 to the terminal of the solenoid 30, whereby the solenoid 30 is energized so as to simultaneously close the switches S and S The closure of switches S and S allows an AC power from the power source 26 to be applied to the electrodes 15 and 15' so as to perform the erasing operation.

When the erasing operation is completed and each of the movable contacts of the switches S and S is moved to contact b, the solenoid 30 is deenergized opening the switches S and S and thus removing the erasing power from the terminals 15 and 15. After the expiration of a time sufficient for the release of the switches S and 8;, each of the movable contact is connected to the contact 0, whereby the high voltage generator 22 is energized again to produce the high voltage power.

It is apparent from the foregoing description that the erasing power source 22 is electrically separated from the high voltage during the whole operation period. It is to be noted that the switches S and S may be operated either manually or automatically in accordance with practical condition.

It is to be understood that the above described arrangement is illustrative of the application of the principle of the invention. Numerous other arrangements may be designed by those skilled in the art without departing from the spirit and scope of the invention. Accordingly, it is to be understood that the present invention is to be limited only by the scope of the appended claims.

What is claimed is:

1. An image storage device having a transparent back plate coated on one side thereof with cathode chromatic material and on the other side with electrically resistive coating, in combination with a circuit arrangement which comprises: A first circuit for applying a high voltage to said back plate; first switch means for deenergizing said first circuit; a second circuit for supplying a current to said resistive coating; and second switch means, operated by said first switch means, for energizing said second circuit while said first circuit is de-energized. 

1. An image storage device having a transparent back plate coated on one side thereof with cathode chromatic material and on the other side with electrically resistive coating, in combination with a circuit arrangement which comprises: A first circuit for applying a high voltage to said back plate; first switch means for deenergizing said first circuit; a second circuit for supplying a current to said resistive coating; and second switch means, operated by said first switch means, for energizing said second circuit while said first circuit is deenergized. 